We recently have demonstrated the feasibility of making measurements of nails in living subjects in situ and in vivo, making it feasible to avoid the need for clipping nails to carry out nail dosimetry, instead making the measurements with a new type of resonator that can make sensitive measurements at a high frequency (9.5 GHz or X-Band) directly on the nail in vivo. We will determine fully the feasibility and capability of measurements with sufficient accuracy and sensitivity within the dose range of interest under the conditions that would be needed to apply in the field and then determine what parameters may affect the dose response relationships, develop methods to cope with these factors, develop a protocol for fast and accurate dosimetry based on measurements in vivo, and then develop criteria for a clonable prototype of a fully functional EPR dosimetry instrument. We propose to make this instrument capable of being operated by non-expert users. The studies will utilize our experience in making measurements in vivo with human subjects for tooth dosimetry and our experience in making EPR measurements in vitro in clipped nails. Because the measurements are based on a physical effect of ionizing radiafion, it is expected that they can be made at any time after the exposure within a window of several weeks. These changes are present immediately after exposure and persist for a period of a few weeks allowing measurements to be immediately taken and analyzed If an event did occur. The changes are also unaffected by factors that are likely to be present in the population after such an event and therefore will be complementary and synergistic to biologically based biodosimetry. The results of the measurements in this project will provide a critical element for the advancement of project 2, advancing understanding of the origins and distributions of the mechanically-induced EPR signals in nails. The instrumental core will provide critical parts of the developments of the resonators and ultimately in the construction of a sensitive X-Band EPR spectrometer for making the measurements in the field. The TBI core will provide essential access to the patient group with nails irradiated in vivo. The instrumental developments and the construction of the prototype instrument will be carried out in collaboration with Cores A,C , and E.